As is known, expendable ceramic cores have been employed in conjunction with the "lost wax" process to produce ceramic shell molds for the investment casting of various types of precision components, such as for example hollow gas turbine engine blades and vanes. In casting these precision components, the ceramic core typically is quite thin in cross-section and may include complex details to form narrow cooling channels, internal support ribs and other features on the cast blade or vane component. The ceramic core must remain physically and chemically stable at high casting temperatures, be relatively non-reactive with the molten metal being cast and be leachable from the casting within a commercially reasonable time. Moreover, the ceramic core must meet stringent dimensional tolerances mandated by users of the component cast around the core.
In attempts to satisfy these requirements, various techniques, such as transfer molding and injection molding, have been developed to make ceramic cores for metal casting applications. Although these techniques have been developed to the degree that satisfactory ceramic cores can be made, they are relatively costly to practice and result in ceramic cores that add considerably to the cost of making the cast metal component.
In particular, transfer molding techniques typically involve molding a mixture of ceramic particulate and an expensive thermosetting binder to desired shape with the binder assisting in molding of the desired shape and retaining of the as-molded core shape until a subsequent firing step to sinter the ceramic core particulate. During this firing process, if present the solids content of the binder material sinters with the ceramic particulate and becomes an integral part of the ceramic article.
The injection molding technique suffers disadvantages in that this technique typically involves injection molding a mixture of ceramic particulate and a relatively high percentage (e.g. 10% to 20% by weight) of a thermoplastic binder, such as wax, subjecting the molded core shape to a lengthy elevated temperature "prebake" treatment (e.g. 24 to 100 hours at 38.degree. C. to 550.degree. C.) in a powder binder-absorbing media to remove some or all of the binder and thereafter firing the core shape to produce a sintered ceramic core. The "prebake" step is not only costly but also essential in wax-based binder systems to avoid slumping, cracking and blistering of the core during the firing (sintering) step.
U.S. Pat. No. 3,222,435 issued Dec. 7, 1965, and U.S. Pat. No. 4,737,237 issued Mar. 29, 1988 describe injection molding techniques for making ceramic cores for use in metal casting wherein the core shape is rigidized in a mold by setting of a binder or gel in a ceramic slurry injected into the mold. In both patents, the molded, rigidized core shape is sintered upon removal from the mold.
Powder molding techniques wherein the desired shape is rigidized in a mold by freezing a particulate slurry injected into the mold are described in U.S. Pat. Nos. 2,893,102 issued Jul. 7, 1959, 2,869,215 issued Jan. 20, 1959 and 2,944,316 issued Jul. 12, 1960. In the first two of these patents, a molding slurry or slip is injected into a mold maintained at a temperature to freeze the slip to desired shape, the frozen shape is dried outside the mold without development of a liquid phase and then the dried shape is sintered. The frozen core shape is dried by either subliming the frozen water from the molded core shape in a vacuum chamber or by enclosing the frozen core shape in a powder drying/supporting agent which not only absorbs moisture from the core shape as it thaws but also supports the core shape to avoid deformation thereof that would otherwise occur during thawing.
In the last of these patents (U.S. Pat. No. 2,869,215), the molding slip is devoid of any viscous binder and is pretreated to remove water in excess of 2%-4% by weight prior to freezing in the injection mold to shape the slip. After removal from the mold, the frozen shape is thawed under conditions which tolerate the total absence of the binder in the molding slip.
It is an object of the invention to provide a method of making shaped articles, especially ceramic cores for use in metal casting, from a slurry mixture of a freezable liquid and particulate material in a manner that is cost effective in terms of materials, equipment and processing steps employed and that overcomes the disadvantages associated with the methods described hereinabove. In particular, the method of the invention eliminates the need for expensive binders in the slurry mixture, eliminates the need for costly treatment of the slurry mixture prior to molding to remove excess liquid binder, eliminates the need for costly "prebake" treatments in a powder packing media or drying treatments in a vacuum chamber prior to sintering and eliminates the need for complex, costly sintering treatments, all without compromising the overall quality and performance of the ceramic core in metal casting.